It has hitherto been known that: particles including a so-called compound semiconductor such as ZnS and CdSe (also including compound semiconductors doped with such as Ag and Mn) (compound semiconductor particles) have various useful functions such as of being able to emit fluorescence by irradiating the particles with ultraviolet rays and/or electron beams as excitation sources or applying a voltage to the particles (e.g. U.S. Pat. No. 5,455,489). In addition, in recent years, as to compound semiconductor particles including such as CdSe and Mn-doped ZnS, it is being verified on a research level and thus becoming clear that, if the particles are more fined to thereby be quantum-sized, then the particles become excellent in various functions, for example, there can be obtained particles having a high luminance of the fluorescence.
However, the compound semiconductor particles generally have a problem such that, if the compound semiconductor particles are quantum-sized, then they become unable to sufficiently perform their peculiar functions, for the reason such that: the particles are lacking in heat resistance, and their surfaces become more easily oxidized. Thus, it is thought that: the particles are lacking in stability and durability and are very difficult to put to practical use. In addition, in the case where the particles are fined to thereby be quantum-sized, it becomes more and more difficult to maintain a favorable monodispersed state. However, unless the compound semiconductor particles are in such a favorable monodispersed state, for example, there is a problem that: their luminosity greatly decreases, and further there is a problem that the properties such as luminescence greatly vary also with the kinds of dispersants as used for dispersing the particles.
On the other hand, as is mentioned above, the dispersed state of the compound semiconductor particles has a great influence on such as their luminosity and/or luminescence efficiency. Thus, in order to treat the compound semiconductor particles in a state where their primary particles are isolated from each other (in the monodispersed state), there have hitherto been made the following proposals.
First, there has been proposed a method that involves treating the surfaces of CdSe particles with ZnS (for example, M. G. Bawendi et al., “J. Phys. Chem. B”, volume 101, pages 9463 to 9475 (published in 1997)). However, as to this method, the compound semiconductor particles which are inherently lacking in chemical durability (e.g. oxidation resistance) and thermal durability are merely treated with the similar compound semiconductor. Therefore, even if the properties are temporarily enhanced, there is a problem of lacking the practicability after all.
In addition, there has been proposed a method that involves attaching oxide particles (e.g. zinc oxide, indium oxide and silica) to surfaces of micron-sized sulfide-type fluophor particles (for example, JP-A-104684/1989 and JP-A-041389/1990). However, as to this method, the attachment of the oxide particles to the compound semiconductor particles needs a heat-treatment step at a high temperature after the attachment-treatment step. Therefore, there are problems such that: the movement of substances and the defectiveness occur during this heating at the high temperature to greatly deteriorate the luminescence property; and there are economical disadvantages of increasing such as treatment costs, and it is necessary to involve a complicated treatment step, so the productivity is inferior.
Furthermore, in recent years, there have been proposed some methods that involve covering a nano-sized level of fluophor particles with such as sulfides, oxides, and other inorganic substances (for example, U.S. Pat. No. 5,985,173, JP-A-265166/2000, Japanese Patent No. 2514423, and Japanese Patent No. 2946763). However, these methods, for example, have the following prior problems: the chemical and thermal stabilities are insufficient; the monodispersibility is so inferior that the excellent fluorescence property as expected by the nano-sizing cannot sufficiently be displayed; and also, the economy is poor, and the productivity is inferior.